In general, a VCO (voltage controlled oscillator) is an oscillator circuit that outputs an AC signal having a frequency that varies in response to an input control voltage. VCOs are fundamental components that are employed in a broad range of applications including radar and communications systems (e.g., wireline or wireless applications) for data transfer and recovery processes. By way of example, VCOs are utilized for PLL (phase locked loop) circuits, DLL (delay locked loop) circuits, or injection locked oscillators. VCOs are further employed for applications such as frequency translation, data modulation, clock distribution and clock/data recovery. There are many different types of VCOs, with one type being an LC-VCO that includes an inductor-capacitor (LC) oscillator (also called a resonator).
Phase noise is instantaneous variations in the frequency of oscillation in LC-VCOs and is related to intrinsic properties of the devices such as: flicker noise and thermal noise of transistors, and the Q factor of inductor-capacitor network. Phase noise can also be caused by various loss elements, such as resistances which may vary with variations of temperature, load, and processing. Phase noise directly affects signal jitter, which is undesirable for clock signal applications.